/**
 * @file
 * DNS - host name to IP address resolver.
 *
 */

/**

 * This file implements a DNS host name to IP address resolver.

 * Port to lwIP from uIP
 * by Jim Pettinato April 2007

 * uIP version Copyright (c) 2002-2003, Adam Dunkels.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior
 *    written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS
 * OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 * GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * DNS.C
 *
 * The lwIP DNS resolver functions are used to lookup a host name and
 * map it to a numerical IP address. It maintains a list of resolved
 * hostnames that can be queried with the dns_lookup() function.
 * New hostnames can be resolved using the dns_query() function.
 *
 * The lwIP version of the resolver also adds a non-blocking version of
 * gethostbyname() that will work with a raw API application. This function
 * checks for an IP address string first and converts it if it is valid.
 * gethostbyname() then does a dns_lookup() to see if the name is
 * already in the table. If so, the IP is returned. If not, a query is
 * issued and the function returns with a ERR_INPROGRESS status. The app
 * using the dns client must then go into a waiting state.
 *
 * Once a hostname has been resolved (or found to be non-existent),
 * the resolver code calls a specified callback function (which
 * must be implemented by the module that uses the resolver).
 */

/*-----------------------------------------------------------------------------
 * RFC 1035 - Domain names - implementation and specification
 * RFC 2181 - Clarifications to the DNS Specification
 *----------------------------------------------------------------------------*/

/** @todo: define good default values (rfc compliance) */
/** @todo: improve answer parsing, more checkings... */
/** @todo: check RFC1035 - 7.3. Processing responses */

/*-----------------------------------------------------------------------------
 * Includes
 *----------------------------------------------------------------------------*/

#include "lwip/opt.h"

#if LWIP_DNS /* don't build if not configured for use in lwipopts.h */

#include "lwip/udp.h"
#include "lwip/mem.h"
#include "lwip/dns.h"

#include <string.h>

/** DNS server IP address */
#ifndef DNS_SERVER_ADDRESS
	#define DNS_SERVER_ADDRESS        inet_addr("208.67.222.222") /* resolver1.opendns.com */
#endif

/** DNS server port address */
#ifndef DNS_SERVER_PORT
	#define DNS_SERVER_PORT           53
#endif

/** DNS maximum number of retries when asking for a name, before "timeout". */
#ifndef DNS_MAX_RETRIES
	#define DNS_MAX_RETRIES           4
#endif

/** DNS resource record max. TTL (one week as default) */
#ifndef DNS_MAX_TTL
	#define DNS_MAX_TTL               604800
#endif

/* DNS protocol flags */
#define DNS_FLAG1_RESPONSE        0x80
#define DNS_FLAG1_OPCODE_STATUS   0x10
#define DNS_FLAG1_OPCODE_INVERSE  0x08
#define DNS_FLAG1_OPCODE_STANDARD 0x00
#define DNS_FLAG1_AUTHORATIVE     0x04
#define DNS_FLAG1_TRUNC           0x02
#define DNS_FLAG1_RD              0x01
#define DNS_FLAG2_RA              0x80
#define DNS_FLAG2_ERR_MASK        0x0f
#define DNS_FLAG2_ERR_NONE        0x00
#define DNS_FLAG2_ERR_NAME        0x03

/* DNS protocol states */
#define DNS_STATE_UNUSED          0
#define DNS_STATE_NEW             1
#define DNS_STATE_ASKING          2
#define DNS_STATE_DONE            3

#ifdef PACK_STRUCT_USE_INCLUDES
	#include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS message header */
struct dns_hdr {
	PACK_STRUCT_FIELD(u16_t id);
	PACK_STRUCT_FIELD(u8_t flags1);
	PACK_STRUCT_FIELD(u8_t flags2);
	PACK_STRUCT_FIELD(u16_t numquestions);
	PACK_STRUCT_FIELD(u16_t numanswers);
	PACK_STRUCT_FIELD(u16_t numauthrr);
	PACK_STRUCT_FIELD(u16_t numextrarr);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
	#include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_HDR 12

#ifdef PACK_STRUCT_USE_INCLUDES
	#include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS query message structure */
struct dns_query {
	/* DNS query record starts with either a domain name or a pointer
	   to a name already present somewhere in the packet. */
	PACK_STRUCT_FIELD(u16_t type);
	PACK_STRUCT_FIELD(u16_t class);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
	#include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_QUERY 4

#ifdef PACK_STRUCT_USE_INCLUDES
	#include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS answer message structure */
struct dns_answer {
	/* DNS answer record starts with either a domain name or a pointer
	   to a name already present somewhere in the packet. */
	PACK_STRUCT_FIELD(u16_t type);
	PACK_STRUCT_FIELD(u16_t class);
	PACK_STRUCT_FIELD(u32_t ttl);
	PACK_STRUCT_FIELD(u16_t len);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
	#include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_ANSWER 10

/** DNS table entry */
struct dns_table_entry {
	u8_t  state;
	u8_t  numdns;
	u8_t  tmr;
	u8_t  retries;
	u8_t  seqno;
	u8_t  err;
	u32_t ttl;
	char name[DNS_MAX_NAME_LENGTH];
	struct ip_addr ipaddr;
	/* pointer to callback on DNS query done */
	dns_found_callback found;
	void* arg;
};

#if DNS_LOCAL_HOSTLIST
/** struct used for local host-list */
struct local_hostlist_entry {
	/** static hostname */
	const char* name;
	/** static host address in network byteorder */
	u32_t addr;
	struct local_hostlist_entry* next;
};

#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
	/** Local host-list. For hostnames in this list, no
	*  external name resolution is performed */
	static struct local_hostlist_entry* local_hostlist_dynamic;
#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

	/** Defining this allows the local_hostlist_static to be placed in a different
	* linker section (e.g. FLASH) */
	#ifndef DNS_LOCAL_HOSTLIST_STORAGE_PRE
		#define DNS_LOCAL_HOSTLIST_STORAGE_PRE static
	#endif /* DNS_LOCAL_HOSTLIST_STORAGE_PRE */
	/** Defining this allows the local_hostlist_static to be placed in a different
	* linker section (e.g. FLASH) */
	#ifndef DNS_LOCAL_HOSTLIST_STORAGE_POST
		#define DNS_LOCAL_HOSTLIST_STORAGE_POST
	#endif /* DNS_LOCAL_HOSTLIST_STORAGE_POST */
	DNS_LOCAL_HOSTLIST_STORAGE_PRE struct local_hostlist_entry local_hostlist_static[]
	DNS_LOCAL_HOSTLIST_STORAGE_POST = DNS_LOCAL_HOSTLIST_INIT;

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */

static void dns_init_local();
#endif /* DNS_LOCAL_HOSTLIST */


/* forward declarations */
static void dns_recv(void* s, struct udp_pcb* pcb, struct pbuf* p, struct ip_addr* addr, u16_t port);
static void dns_check_entries(void);

/*-----------------------------------------------------------------------------
 * Globales
 *----------------------------------------------------------------------------*/

/* DNS variables */
static struct udp_pcb*        dns_pcb;
static u8_t                   dns_seqno;
static struct dns_table_entry dns_table[DNS_TABLE_SIZE];
static struct ip_addr         dns_servers[DNS_MAX_SERVERS];

#if (DNS_USES_STATIC_BUF == 1)
	static u8_t                   dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 1) */

/**
 * Initialize the resolver: set up the UDP pcb and configure the default server
 * (DNS_SERVER_ADDRESS).
 */
void
dns_init()
{
	struct ip_addr dnsserver;

	/* initialize default DNS server address */
	dnsserver.addr = DNS_SERVER_ADDRESS;

	LWIP_DEBUGF(DNS_DEBUG, ("dns_init: initializing\n"));

	/* if dns client not yet initialized... */
	if(dns_pcb == NULL) {
		dns_pcb = udp_new();

		if(dns_pcb != NULL) {
			/* initialize DNS table not needed (initialized to zero since it is a
			 * global variable) */
			LWIP_ASSERT("For implicit initialization to work, DNS_STATE_UNUSED needs to be 0",
			            DNS_STATE_UNUSED == 0);

			/* initialize DNS client */
			udp_bind(dns_pcb, IP_ADDR_ANY, 0);
			udp_recv(dns_pcb, dns_recv, NULL);

			/* initialize default DNS primary server */
			dns_setserver(0, &dnsserver);
		}
	}

#if DNS_LOCAL_HOSTLIST
	dns_init_local();
#endif
}

/**
 * Initialize one of the DNS servers.
 *
 * @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS
 * @param dnsserver IP address of the DNS server to set
 */
void
dns_setserver(u8_t numdns, struct ip_addr* dnsserver)
{
	if((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) &&
	        (dnsserver != NULL) && (dnsserver->addr != 0)) {
		dns_servers[numdns] = (*dnsserver);
	}
}

/**
 * Obtain one of the currently configured DNS server.
 *
 * @param numdns the index of the DNS server
 * @return IP address of the indexed DNS server or "ip_addr_any" if the DNS
 *         server has not been configured.
 */
struct ip_addr
dns_getserver(u8_t numdns)
{
	if(numdns < DNS_MAX_SERVERS) {
		return dns_servers[numdns];
	} else {
		return *IP_ADDR_ANY;
	}
}

/**
 * The DNS resolver client timer - handle retries and timeouts and should
 * be called every DNS_TMR_INTERVAL milliseconds (every second by default).
 */
void
dns_tmr(void)
{
	if(dns_pcb != NULL) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_tmr: dns_check_entries\n"));
		dns_check_entries();
	}
}

#if DNS_LOCAL_HOSTLIST
static void
dns_init_local()
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT)
	int i;
	struct local_hostlist_entry* entry;
	/* Dynamic: copy entries from DNS_LOCAL_HOSTLIST_INIT to list */
	struct local_hostlist_entry local_hostlist_init[] = DNS_LOCAL_HOSTLIST_INIT;

	for(i = 0; i < sizeof(local_hostlist_init) / sizeof(struct local_hostlist_entry); i++) {
		entry = mem_malloc(sizeof(struct local_hostlist_entry));
		LWIP_ASSERT("mem-error in dns_init_local", entry != NULL);

		if(entry != NULL) {
			struct local_hostlist_entry* init_entry = &local_hostlist_init[i];
			entry->name = init_entry->name;
			entry->addr = init_entry->addr;
			entry->next = local_hostlist_dynamic;
			local_hostlist_dynamic = entry;
		}
	}

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT) */
}

/**
 * Scans the local host-list for a hostname.
 *
 * @param hostname Hostname to look for in the local host-list
 * @return The first IP address for the hostname in the local host-list or
 *         INADDR_NONE if not found.
 */
static u32_t
dns_lookup_local(const char* hostname)
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
	struct local_hostlist_entry* entry = local_hostlist_dynamic;

	while(entry != NULL) {
		if(strcmp(entry->name, hostname) == 0) {
			return entry->addr;
		}

		entry = entry->next;
	}

#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
	int i;

	for(i = 0; i < sizeof(local_hostlist_static) / sizeof(struct local_hostlist_entry); i++) {
		if(strcmp(local_hostlist_static[i].name, hostname) == 0) {
			return local_hostlist_static[i].addr;
		}
	}

#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
	return INADDR_NONE;
}

#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/** Remove all entries from the local host-list for a specific hostname
 * and/or IP addess
 *
 * @param hostname hostname for which entries shall be removed from the local
 *                 host-list
 * @param addr address for which entries shall be removed from the local host-list
 * @return the number of removed entries
 */
int
dns_local_removehost(const char* hostname, const struct ip_addr* addr)
{
	int removed = 0;
	struct local_hostlist_entry* entry = local_hostlist_dynamic;
	struct local_hostlist_entry* last_entry = NULL;

	while(entry != NULL) {
		if(((hostname == NULL) || !strcmp(entry->name, hostname)) &&
		        ((addr == NULL) || (entry->addr == addr->addr))) {
			struct local_hostlist_entry* free_entry;

			if(last_entry != NULL) {
				last_entry->next = entry->next;
			} else {
				local_hostlist_dynamic = entry->next;
			}

			free_entry = entry;
			entry = entry->next;
			mem_free(free_entry);
			removed++;
		} else {
			last_entry = entry;
			entry = entry->next;
		}
	}

	return removed;
}

/**
 * Add a hostname/IP address pair to the local host-list.
 * Duplicates are not checked.
 *
 * @param hostname hostname of the new entry
 * @param addr IP address of the new entry
 * @return ERR_OK if succeeded or ERR_MEM on memory error
 */
err_t
dns_local_addhost(const char* hostname, const struct ip_addr* addr)
{
	struct local_hostlist_entry* entry;
	entry = mem_malloc(sizeof(struct local_hostlist_entry));

	if(entry == NULL) {
		return ERR_MEM;
	}

	entry->name = hostname;
	entry->addr = addr->addr;
	entry->next = local_hostlist_dynamic;
	local_hostlist_dynamic = entry;
	return ERR_OK;
}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC*/
#endif /* DNS_LOCAL_HOSTLIST */

/**
 * Look up a hostname in the array of known hostnames.
 *
 * @note This function only looks in the internal array of known
 * hostnames, it does not send out a query for the hostname if none
 * was found. The function dns_enqueue() can be used to send a query
 * for a hostname.
 *
 * @param name the hostname to look up
 * @return the hostname's IP address, as u32_t (instead of struct ip_addr to
 *         better check for failure: != INADDR_NONE) or INADDR_NONE if the hostname
 *         was not found in the cached dns_table.
 */
static u32_t
dns_lookup(const char* name)
{
	u8_t i;
#if DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN)
	u32_t addr;
#endif /* DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN) */
#if DNS_LOCAL_HOSTLIST

	if((addr = dns_lookup_local(name)) != INADDR_NONE) {
		return addr;
	}

#endif /* DNS_LOCAL_HOSTLIST */
#ifdef DNS_LOOKUP_LOCAL_EXTERN

	if((addr = DNS_LOOKUP_LOCAL_EXTERN(name)) != INADDR_NONE) {
		return addr;
	}

#endif /* DNS_LOOKUP_LOCAL_EXTERN */

	/* Walk through name list, return entry if found. If not, return NULL. */
	for(i = 0; i < DNS_TABLE_SIZE; ++i) {
		if((dns_table[i].state == DNS_STATE_DONE) &&
		        (strcmp(name, dns_table[i].name) == 0)) {
			LWIP_DEBUGF(DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
			ip_addr_debug_print(DNS_DEBUG, &(dns_table[i].ipaddr));
			LWIP_DEBUGF(DNS_DEBUG, ("\n"));
			return dns_table[i].ipaddr.addr;
		}
	}

	return INADDR_NONE;
}

#if DNS_DOES_NAME_CHECK
/**
 * Compare the "dotted" name "query" with the encoded name "response"
 * to make sure an answer from the DNS server matches the current dns_table
 * entry (otherwise, answers might arrive late for hostname not on the list
 * any more).
 *
 * @param query hostname (not encoded) from the dns_table
 * @param response encoded hostname in the DNS response
 * @return 0: names equal; 1: names differ
 */
static u8_t
dns_compare_name(unsigned char* query, unsigned char* response)
{
	unsigned char n;

	do {
		n = *response++;

		/** @see RFC 1035 - 4.1.4. Message compression */
		if((n & 0xc0) == 0xc0) {
			/* Compressed name */
			break;
		} else {
			/* Not compressed name */
			while(n > 0) {
				if((*query) != (*response)) {
					return 1;
				}

				++response;
				++query;
				--n;
			};

			++query;
		}
	} while(*response != 0);

	return 0;
}
#endif /* DNS_DOES_NAME_CHECK */

/**
 * Walk through a compact encoded DNS name and return the end of the name.
 *
 * @param query encoded DNS name in the DNS server response
 * @return end of the name
 */
static unsigned char*
dns_parse_name(unsigned char* query)
{
	unsigned char n;

	do {
		n = *query++;

		/** @see RFC 1035 - 4.1.4. Message compression */
		if((n & 0xc0) == 0xc0) {
			/* Compressed name */
			break;
		} else {
			/* Not compressed name */
			while(n > 0) {
				++query;
				--n;
			};
		}
	} while(*query != 0);

	return query + 1;
}

/**
 * Send a DNS query packet.
 *
 * @param numdns index of the DNS server in the dns_servers table
 * @param name hostname to query
 * @param id index of the hostname in dns_table, used as transaction ID in the
 *        DNS query packet
 * @return ERR_OK if packet is sent; an err_t indicating the problem otherwise
 */
static err_t
dns_send(u8_t numdns, const char* name, u8_t id)
{
	err_t err;
	struct dns_hdr* hdr;
	struct dns_query qry;
	struct pbuf* p;
	char* query, *nptr;
	const char* pHostname;
	u8_t n;

	LWIP_DEBUGF(DNS_DEBUG, ("dns_send: dns_servers[%"U16_F"] \"%s\": request\n",
	                        (u16_t)(numdns), name));
	LWIP_ASSERT("dns server out of array", numdns < DNS_MAX_SERVERS);
	LWIP_ASSERT("dns server has no IP address set", dns_servers[numdns].addr != 0);

	/* if here, we have either a new query or a retry on a previous query to process */
	p = pbuf_alloc(PBUF_TRANSPORT, SIZEOF_DNS_HDR + DNS_MAX_NAME_LENGTH +
	               SIZEOF_DNS_QUERY, PBUF_RAM);

	if(p != NULL) {
		LWIP_ASSERT("pbuf must be in one piece", p->next == NULL);
		/* fill dns header */
		hdr = (struct dns_hdr*)p->payload;
		memset(hdr, 0, SIZEOF_DNS_HDR);
		hdr->id = htons(id);
		hdr->flags1 = DNS_FLAG1_RD;
		hdr->numquestions = htons(1);
		query = (char*)hdr + SIZEOF_DNS_HDR;
		pHostname = name;
		--pHostname;

		/* convert hostname into suitable query format. */
		do {
			++pHostname;
			nptr = query;
			++query;

			for(n = 0; *pHostname != '.' && *pHostname != 0; ++pHostname) {
				*query = *pHostname;
				++query;
				++n;
			}

			*nptr = n;
		} while(*pHostname != 0);

		*query++ = '\0';

		/* fill dns query */
		qry.type  = htons(DNS_RRTYPE_A);
		qry.class = htons(DNS_RRCLASS_IN);
		MEMCPY(query, &qry, SIZEOF_DNS_QUERY);

		/* resize pbuf to the exact dns query */
		pbuf_realloc(p, (query + SIZEOF_DNS_QUERY) - ((char*)(p->payload)));

		/* connect to the server for faster receiving */
		udp_connect(dns_pcb, &dns_servers[numdns], DNS_SERVER_PORT);
		/* send dns packet */
		err = udp_sendto(dns_pcb, p, &dns_servers[numdns], DNS_SERVER_PORT);

		/* free pbuf */
		pbuf_free(p);
	} else {
		err = ERR_MEM;
	}

	return err;
}

/**
 * dns_check_entry() - see if pEntry has not yet been queried and, if so, sends out a query.
 * Check an entry in the dns_table:
 * - send out query for new entries
 * - retry old pending entries on timeout (also with different servers)
 * - remove completed entries from the table if their TTL has expired
 *
 * @param i index of the dns_table entry to check
 */
static void
dns_check_entry(u8_t i)
{
	struct dns_table_entry* pEntry = &dns_table[i];

	LWIP_ASSERT("array index out of bounds", i < DNS_TABLE_SIZE);

	switch(pEntry->state) {

		case DNS_STATE_NEW: {
			/* initialize new entry */
			pEntry->state   = DNS_STATE_ASKING;
			pEntry->numdns  = 0;
			pEntry->tmr     = 1;
			pEntry->retries = 0;

			/* send DNS packet for this entry */
			dns_send(pEntry->numdns, pEntry->name, i);
			break;
		}

		case DNS_STATE_ASKING: {
			if(--pEntry->tmr == 0) {
				if(++pEntry->retries == DNS_MAX_RETRIES) {
					if((pEntry->numdns + 1 < DNS_MAX_SERVERS) && (dns_servers[pEntry->numdns + 1].addr != 0)) {
						/* change of server */
						pEntry->numdns++;
						pEntry->tmr     = 1;
						pEntry->retries = 0;
						break;
					} else {
						LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": timeout\n", pEntry->name));

						/* call specified callback function if provided */
						if(pEntry->found)
							(*pEntry->found)(pEntry->name, NULL, pEntry->arg);

						/* flush this entry */
						pEntry->state   = DNS_STATE_UNUSED;
						pEntry->found   = NULL;
						break;
					}
				}

				/* wait longer for the next retry */
				pEntry->tmr = pEntry->retries;

				/* send DNS packet for this entry */
				dns_send(pEntry->numdns, pEntry->name, i);
			}

			break;
		}

		case DNS_STATE_DONE: {
			/* if the time to live is nul */
			if(--pEntry->ttl == 0) {
				LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
				/* flush this entry */
				pEntry->state = DNS_STATE_UNUSED;
				pEntry->found = NULL;
			}

			break;
		}

		case DNS_STATE_UNUSED:
			/* nothing to do */
			break;

		default:
			LWIP_ASSERT("unknown dns_table entry state:", 0);
			break;
	}
}

/**
 * Call dns_check_entry for each entry in dns_table - check all entries.
 */
static void
dns_check_entries(void)
{
	u8_t i;

	for(i = 0; i < DNS_TABLE_SIZE; ++i) {
		dns_check_entry(i);
	}
}

/**
 * Receive input function for DNS response packets arriving for the dns UDP pcb.
 *
 * @params see udp.h
 */
static void
dns_recv(void* arg, struct udp_pcb* pcb, struct pbuf* p, struct ip_addr* addr, u16_t port)
{
	u8_t i;
	char* pHostname;
	struct dns_hdr* hdr;
	struct dns_answer ans;
	struct dns_table_entry* pEntry;
	u8_t nquestions, nanswers;
#if (DNS_USES_STATIC_BUF == 0)
	u8_t dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 0) */
#if (DNS_USES_STATIC_BUF == 2)
	u8_t* dns_payload;
#endif /* (DNS_USES_STATIC_BUF == 2) */

	LWIP_UNUSED_ARG(arg);
	LWIP_UNUSED_ARG(pcb);
	LWIP_UNUSED_ARG(addr);
	LWIP_UNUSED_ARG(port);

	/* is the dns message too big ? */
	if(p->tot_len > DNS_MSG_SIZE) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too big\n"));
		/* free pbuf and return */
		goto memerr1;
	}

	/* is the dns message big enough ? */
	if(p->tot_len < (SIZEOF_DNS_HDR + SIZEOF_DNS_QUERY + SIZEOF_DNS_ANSWER)) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
		/* free pbuf and return */
		goto memerr1;
	}

#if (DNS_USES_STATIC_BUF == 2)
	dns_payload = mem_malloc(p->tot_len);

	if(dns_payload == NULL) {
		LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: mem_malloc error\n"));
		/* free pbuf and return */
		goto memerr1;
	}

#endif /* (DNS_USES_STATIC_BUF == 2) */

	/* copy dns payload inside static buffer for processing */
	if(pbuf_copy_partial(p, dns_payload, p->tot_len, 0) == p->tot_len) {
		/* The ID in the DNS header should be our entry into the name table. */
		hdr = (struct dns_hdr*)dns_payload;
		i = htons(hdr->id);

		if(i < DNS_TABLE_SIZE) {
			pEntry = &dns_table[i];

			if(pEntry->state == DNS_STATE_ASKING) {
				/* This entry is now completed. */
				pEntry->state = DNS_STATE_DONE;
				pEntry->err   = hdr->flags2 & DNS_FLAG2_ERR_MASK;

				/* We only care about the question(s) and the answers. The authrr
				   and the extrarr are simply discarded. */
				nquestions = htons(hdr->numquestions);
				nanswers   = htons(hdr->numanswers);

				/* Check for error. If so, call callback to inform. */
				if(((hdr->flags1 & DNS_FLAG1_RESPONSE) == 0) || (pEntry->err != 0) || (nquestions != 1)) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in flags\n", pEntry->name));
					/* call callback to indicate error, clean up memory and return */
					goto responseerr;
				}

#if DNS_DOES_NAME_CHECK

				/* Check if the name in the "question" part match with the name in the entry. */
				if(dns_compare_name((unsigned char*)(pEntry->name), (unsigned char*)dns_payload + SIZEOF_DNS_HDR) != 0) {
					LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", pEntry->name));
					/* call callback to indicate error, clean up memory and return */
					goto responseerr;
				}

#endif /* DNS_DOES_NAME_CHECK */

				/* Skip the name in the "question" part */
				pHostname = (char*) dns_parse_name((unsigned char*)dns_payload + SIZEOF_DNS_HDR) + SIZEOF_DNS_QUERY;

				while(nanswers > 0) {
					/* skip answer resource record's host name */
					pHostname = (char*) dns_parse_name((unsigned char*)pHostname);

					/* Check for IP address type and Internet class. Others are discarded. */
					MEMCPY(&ans, pHostname, SIZEOF_DNS_ANSWER);

					if((ntohs(ans.type) == DNS_RRTYPE_A) && (ntohs(ans.class) == DNS_RRCLASS_IN) && (ntohs(ans.len) == sizeof(struct ip_addr))) {
						/* read the answer resource record's TTL, and maximize it if needed */
						pEntry->ttl = ntohl(ans.ttl);

						if(pEntry->ttl > DNS_MAX_TTL) {
							pEntry->ttl = DNS_MAX_TTL;
						}

						/* read the IP address after answer resource record's header */
						MEMCPY(&(pEntry->ipaddr), (pHostname + SIZEOF_DNS_ANSWER), sizeof(struct ip_addr));
						LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response = ", pEntry->name));
						ip_addr_debug_print(DNS_DEBUG, (&(pEntry->ipaddr)));
						LWIP_DEBUGF(DNS_DEBUG, ("\n"));

						/* call specified callback function if provided */
						if(pEntry->found) {
							(*pEntry->found)(pEntry->name, &pEntry->ipaddr, pEntry->arg);
						}

						/* deallocate memory and return */
						goto memerr2;
					} else {
						pHostname = pHostname + SIZEOF_DNS_ANSWER + htons(ans.len);
					}

					--nanswers;
				}

				LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in response\n", pEntry->name));
				/* call callback to indicate error, clean up memory and return */
				goto responseerr;
			}
		}
	}

	/* deallocate memory and return */
	goto memerr2;

responseerr:

	/* ERROR: call specified callback function with NULL as name to indicate an error */
	if(pEntry->found) {
		(*pEntry->found)(pEntry->name, NULL, pEntry->arg);
	}

	/* flush this entry */
	pEntry->state = DNS_STATE_UNUSED;
	pEntry->found = NULL;

memerr2:
#if (DNS_USES_STATIC_BUF == 2)
	/* free dns buffer */
	mem_free(dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */

memerr1:
	/* free pbuf */
	pbuf_free(p);
	return;
}

/**
 * Queues a new hostname to resolve and sends out a DNS query for that hostname
 *
 * @param name the hostname that is to be queried
 * @param found a callback founction to be called on success, failure or timeout
 * @param callback_arg argument to pass to the callback function
 * @return @return a err_t return code.
 */
static err_t
dns_enqueue(const char* name, dns_found_callback found, void* callback_arg)
{
	u8_t i;
	u8_t lseq, lseqi;
	struct dns_table_entry* pEntry = NULL;

	/* search an unused entry, or the oldest one */
	lseq = lseqi = 0;

	for(i = 0; i < DNS_TABLE_SIZE; ++i) {
		pEntry = &dns_table[i];

		/* is it an unused entry ? */
		if(pEntry->state == DNS_STATE_UNUSED)
			break;

		/* check if this is the oldest completed entry */
		if(pEntry->state == DNS_STATE_DONE) {
			if((dns_seqno - pEntry->seqno) > lseq) {
				lseq = dns_seqno - pEntry->seqno;
				lseqi = i;
			}
		}
	}

	/* if we don't have found an unused entry, use the oldest completed one */
	if(i == DNS_TABLE_SIZE) {
		if((lseqi >= DNS_TABLE_SIZE) || (dns_table[lseqi].state != DNS_STATE_DONE)) {
			/* no entry can't be used now, table is full */
			LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": DNS entries table is full\n", name));
			return ERR_MEM;
		} else {
			/* use the oldest completed one */
			i = lseqi;
			pEntry = &dns_table[i];
		}
	}

	/* use this entry */
	LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": use DNS entry %"U16_F"\n", name, (u16_t)(i)));

	/* fill the entry */
	pEntry->state = DNS_STATE_NEW;
	pEntry->seqno = dns_seqno++;
	pEntry->found = found;
	pEntry->arg   = callback_arg;
	strcpy(pEntry->name, name);

	/* force to send query without waiting timer */
	dns_check_entry(i);

	/* dns query is enqueued */
	return ERR_INPROGRESS;
}

/**
 * Resolve a hostname (string) into an IP address.
 * NON-BLOCKING callback version for use with raw API!!!
 *
 * Returns immediately with one of err_t return codes:
 * - ERR_OK if hostname is a valid IP address string or the host
 *   name is already in the local names table.
 * - ERR_INPROGRESS enqueue a request to be sent to the DNS server
 *   for resolution if no errors are present.
 *
 * @param hostname the hostname that is to be queried
 * @param addr pointer to a struct ip_addr where to store the address if it is already
 *             cached in the dns_table (only valid if ERR_OK is returned!)
 * @param found a callback function to be called on success, failure or timeout (only if
 *              ERR_INPROGRESS is returned!)
 * @param callback_arg argument to pass to the callback function
 * @return a err_t return code.
 */
err_t
dns_gethostbyname(const char* hostname, struct ip_addr* addr, dns_found_callback found,
                  void* callback_arg)
{
	/* not initialized or no valid server yet, or invalid addr pointer
	 * or invalid hostname or invalid hostname length */
	if((dns_pcb == NULL) || (addr == NULL) ||
	        (!hostname) || (!hostname[0]) ||
	        (strlen(hostname) >= DNS_MAX_NAME_LENGTH)) {
		return ERR_VAL;
	}

#if LWIP_HAVE_LOOPIF

	if(strcmp(hostname, "localhost") == 0) {
		addr->addr = htonl(INADDR_LOOPBACK);
		return ERR_OK;
	}

#endif /* LWIP_HAVE_LOOPIF */

	/* host name already in octet notation? set ip addr and return ERR_OK
	 * already have this address cached? */
	if(((addr->addr = inet_addr(hostname)) != INADDR_NONE) ||
	        ((addr->addr = dns_lookup(hostname)) != INADDR_NONE)) {
		return ERR_OK;
	}

	/* queue query with specified callback */
	return dns_enqueue(hostname, found, callback_arg);
}

#endif /* LWIP_DNS */
